The invention is concerned with an electronic device for the detection and elimination of unwanted images created by the pyramidal IR dome of an infra-red auto-director (ADIR) for a missile which is not roll-stabilized, the signal collected in the focal plane of the objective of the auto-director and originating, either in the case of a first type, in a source located in the field of the said objective and whose radiation is transmitted through the IR dome, or in the case of a second type, in a source, external to the said field and whose radiation after transmission through a facet of the IR dome is reflected by the opposite facet.
The source located in the field of the objective (the origin of the first type) may be the useful target, an element of the landscape, a piece of "window", etc.
The source external to the field of the objective (the origin of the second type) may be an element of the landscape (including the sun), a piece of "window", etc.
The images which it is proposed here to eliminate are those corresponding to an origin of the second kind, whether they are elements of the landscape or pieces of "window", distinguishing between elements of landscapes and "window" not being the mask of the said device.
The theoretical remedy for the non-creation of such unwanted images consists in carrying out on the outer surface of each facet of the IR dome an anti-reflection treatment similar to that which is carried out on the inside surfaces. In practice, this remedy is ineffective owing to the fact that the treatments known to date do not stand up to the conventional conditions to which the IR dome is subjected.
The solution proposed is not an optical aid but an electronic device, the purpose of which is to detect and hence to eliminate these unwanted images.
This device is characterized in that the analysis of the scene delimited by the field of the objective having as its result a succession of frames of period T, the levels of the points on the frame of rank N-1 preceding any frame of rank N are stored in a frame memory, the levels of two points corresponding to the same geographical localization of the scene, one on frame N at an instant t, the other on frame N-1 at an instant t-T, being transmitted respectively to the non-inverting input and to the inverting input of a differential detector whose output is connected via an absolute-value measuring unit to an input of a comparator to whose other input is applied a threshold level with a positive value, the level emerging from the said measuring unit indicating the occurrence of an unwanted image if its value is greater than the said threshold value, the signal at the output of the comparator being used to inhibit the taking into account of the said unwanted image.
As has already been mentioned in the preamble, this device is intended for a missile which is not roll-stabilized. In other words it is an aid to picking-up in flight, renewed pick-up in flight and pursuit.
Provided the elementary duration of processing is limited to a sufficiently low value (e.g. a frame period of 5 ms) such that it is possible to ignore particularly the displacement of the missile (3.5 m at Mach 2), the relative target-missile displacement, and the enlargement of the target (less than an elementary field of 0.5 m radius for a target of diameter 2 m at the very short distance of 100 m), the images of the first type will remain identical, whereas the images of the second type will describe a trajectory during the same time.